Controllable heat pipe



June 30, 1970 T. WYATT 3,517,730

\ )L wd HEAT INPUT ZONE /7Y /4 /8 HEAT OUTPUT ZONE Filed March l5, 1967 /a HEAT OUTPUT ZONE /8 HEAT OUTPUT ZONE THEODORE WYATT INVENTOR June 30, 1970 T WYATT 3,517,730

CONTROLLABLE HEAT PIPE Filed March l5, 1967 8 SheeLS-Sl'leekl 2 THEODORE WYATT INVENTOR June 30, 1970 T. WYATT 3,517,730

CONTROLLABLE HEAT PIPE Filed March 15, 1967 8 Sheets-Sheet 3 INVENT OR THEODORE F/G. 6 wYATT 8 ShQets-Sheet 4 Filed March l5, 1967 L E U F CAPSULE THEODORE WYATT INVENTOR June 30, 1970 T. WYATT CONTROLLABLE HEAT PIPE Filed March 15, 1967 8 Sheets-Sheet 5 RECEVER THEODORE WYATT June 30, 1970 T, WYATT 3,517,730

CONTROLLABLE HEAT PIPE Filed March 15, 1967 8 Sheets-Sheet M f fl 1U MA f Il THEoDoRE wYATT INVENTOR June 30, 1970 T. WYATT CONTROLLABLE HEAT PIPE 8 Sheets-Sheet 7 Filed March 15, 19e? INVENTOR THEODORE WYATT June 30, 1970 T, WYATT GONTROLLABLE HEAT PIPE 8 Sheets-Sheet 8 Filed March l5, 1967 THEODORE WYATT INVENTOR IUnited States Patent Oice 3,517,730 Patented June 30, 1970 3,517,730 CONTROLLABLE HEAT PIPE Theodore Wyatt, Union Bridge, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Mar. 15, 1967, Ser. No. 624,657 Int. Cl. F28d 15/00; G21h 1/10 U.S. Cl. 16S-32 23 Claims ABSTRACT OF THE DISCLOSURE The object of the present invention is to provide apparatus for controlling the temperature within a space vehicle such as an earth satellite. The invention utilizes a heat pipe having a portion within the satellite and a portion extending exteriorly thereof, and having condensable and non-condensable gasses therein. The condensable gas is vaporized by heat from within the satellite and forced by pressure into the portion of the pipe that extends exteriorly of the satellite, at the same time forcing the non-condensable gas into a bellows (or cylinder) connected to the outer end of the pipe. Vapor reaching the exterior portion of the pipe conducts heat from the satellite into free space. Vapor in the exterior portion of the pipe is condensed and returned to the interior portion thereof by a wick. The bellows may be compressed (or a piston in the cylinder moved), by remotely controlled means, for forcing non-condensable gas into the pipe for limiting vapor flow and thus heat discharge from the satellite.

In modified embodiments the invention is used for controlling the output of one or more thermoelectric generators.

This invention relates generally to heat transfer devices of the type known as heat pipes. More particularly it pertains to an improved heat pipe having a controllable output.

Heat pipes, in their broadest aspects, are well-known and have been found to be quite useful for disposing of waste heat in space vehicles such as earth satellites. One such heat pipe is shown and described in my U.S. Pat. No. 3,152,774, assigned to the U.S. Government. In heat pipes in use up to the present time, however, no means has been employed for controlling their outputs.

An important object of the present invention, therefore, is to provide a heat pipe having simple and highly efiicient means for controlling thermal flow therethrough without significantly varying the temperature level at which the heat is transferred and Without requiring any variation of the quantity of heat or the temperature of the heat input source.

Another object of the invention resides in the provision of a controllable heat pipe by the use of which it will be possible to maintain a uniform temperature in the environment representative of either the input or the output end of the pipe, or the difference between them, or the difference between either end of said pipe and a reference temperature.

As a further object the invention provides a heat pipe which lends itself particularly well for use with nuclear power supplies, for transferring heat produced thereby selectively and controllably to one or more apparatuses provided for the conversion of heat vto electricity.

Other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagrammatic view showing a heat pipe in an equilibrium condition;

FIG. 2 is a diagrammatic view illustrating a heat pipe with a bellows at the output end thereof for controlling heat output, the view showing the bellows contracted, for providing minimum heat conduction;

`F-IG. 3 is a view similar to FIG. 2 but showing the bellows extended for providing maximum heat conduction;

FIG. 4 is a detail longitudinal section, partly in elevation, showing one embodiment of a bellows and its associated mounting structure;

FIG. 5 is a side elevation, partly in section illustrating the invention as applied to a satellite;

FIG. 6 is a bottom view of the invention shown in FIG. 5;

FIG. 7 is a diagrammatic view showing a modification of the invention, as used in a space vehicle employing waste heat from a radioactive isotope thermoelectric generator;

FIG. 8 is a diagrammatic view of a further modification of the invention, utilizing heat from a radioactive isotope fuel capsule to activate alternatively either of a pair of thermoelectric generators;

FIG. 9 is a diagrammatic view showing a modified means for varying the position of the non-condensable gas employed for controlling heat flow;

FIG. l0 is a fragmentary diagrammatic view showing a modified version of the invention illustrated in FIG. 8;

FIG. 11 is a fragmentary diagrammatic View showing a further modification of the invention of FIG. 8;

FIG. 12 is a detail diagrammatic view showing still another modified form of the invention of FIG. 8;

FIG. 13 is a diagrammatic view showing a still further modified embodiment of the invention; and

FIG. 14 is a section on the line 14-14 of FIG. 8.

It is well-known in steam power plant practice that, if a non-condensable gas (commonly air dissolved in the feedwater) is admitted to the boiler or any other part of the system, the non-condensable gas is concentrated within the steam condenser. The effect of this accumulation within the condenser is to reduce the heat transfer accomplished within the condenser and is evidenced by a reduction in temperature in that portion of the condenser occupied by the non-condensable gas. Consequently, means are commonly provided to remove the non-condensable gas so as to maintain the heat transfer at the desired maximum rate.

With knowledge of the above-mentioned steam plant characteristic and of the similarity between a steam plant and a heat pipe, it became evident that by varying the amount of non-condensable gas present in a heat pipe the heat transfer accomplished might be controlled.

In addition, an experiment was reported wherein sodium was employed as the condensable gas or working fluid. After noting a temperature discontinuity in the heat output zone the explanation was offered that hydrogen gas was unintentionally present. Under the conditions of the experiment hydrogen would have been a non-condensable gas.

Thereafter an experiment was performed using ethyl alcohol as the working fluid and air as the non-condensable gas. The expected temeperature discontinuity in the heat output zone and reduction in heat fiow was found to be present, as compared to the same heat pipe when operated under identical conditions except with the air absent.

The following explanation is postulated. Assume an inoperative heat pipe containing a working fluid and a gas which are chemically inert with respect to each other and their surroundings and differing greatly in their boiling points-the gas having the lower and being well above its dew point when the heat pipe is operating. The gas and the vapor of the fluid will be uniformly distributed and mixed throughout the heat pipe. If a constant amount of heat is now applied at one end of the heat pipe and withdrawn at the other end an equilibrium in temperature and pressure will be attained within the heat pipe. Liquid will be evaporated at the heat input zone and the resulting vapor will flow to the heat output zone, where the vapor will be condensed to the liquid state. The liquid thus produced will flow by capillary action through a wick back to the heat input zone. The continuing flow of vapor, always being in the same direction, sweeps essentially all of the non-condensable gas to the heat output zone and continually returns any gas molecules tending to migrate from this zone. FIG. l illustrates diagrammatically the equilibrium situation thus created. The non-condensable gas is confined to that portion of the heat output Zone most remote from the heat input zone and gradually loses heat through the surrounding heat output zone. Consequently, the gas is cooler than the vapor and the portion of the heat pipe occupied by gas has a lower temperature than the portion occupied by vapor. Furthermore, the heat extracted from the output zone is reduced in proportion to the extent that the zone is occupied by non-condensable gas.

Thus the experimental observations and the postulated explanation seem to agree.

From the above it will be understood that thermal flow through a heat pipe can be controlled by varying the amount of non-condensable gas present. Such control may be effected reversibly by introducing or withdrawing the non-condensable control gas, as by a bellows or a piston. Various manual or automatic means may be used to adjust the position of the bellows or piston and hence the heat flow through the pipe. A suitable automatic device for, say, satellite temperature control would be the Vernatherm manufactured by the American Radiator and Standard Sanitary Corporation. An example wherein the Vernatherm would be useful is a satellite with internal heat dissipation, as from electrical loads or nuclear decay, sufficiently large compared to the solar input that regulation of the flow of such internal heat to surfaces substantially shielded from the sun, e.g., the base plate of a gravity stabilized satellite, and radiating to space would permit maintenance of a desired nearly constant internal temperature.

It should be understood that the internal pressure of a heat pipe is merely the vapor pressure of the working liquid at the temperature of the input zone. The same pressure prevails throughout, except for small flow losses, and, since the vapor and liquid phases of the working fluid are in temperature-pressure equilibrium throughout, the temperatures in all portions occupied by the condensable vapor are essentially the same. Furthermore, in the applications described herein the temperature of the heat input zone is very nearly the same as the temperature of the source of the heat. As a consequence, even though the operation of the bellows, or cylinder and piston, changes the internal volume of the heat pipe, there is no associated change in the internal pressure and temperature of the heat pipe.

Referring now more particularly to the drawings, and first to FIG. 1 thereof, a basic heat pipe according to the invention is shown. The heat pipe includes a cylindrical pipe section having a side wall 11 which is closed at both ends by walls 12 and 13 and includes a sleeve 14 of suitable wicking material. The sleeve 14 extends throughout the length of the pipe section and engages the inner surface of the side wall 11.

The heat pipe contains a non-condensable gas, such as air or hydrogen, indicated at 15, and a condsenable working fluid such as water or ethyl alcohol, shown at 16. The portion of the pipe shown within the bracket 17 is the heat input zone and is normally located near a source of heat, such as the interior of a space satellite, whereas the portion of the pipe within the bracket 18 is the heat output zone and is located in a position to discharge heat produced, say, within the satellite by radiation to space.

As heat is applied to the pipe section 10 at the input zone 17 thereof, the working fluid 16 is caused to boil and the resulting vaporflows toward the heat output zone 18 for discharge therefrom. As pointed out hereinabove, as long as heat is supplied to the input zone 17, any molecules of the non-condensable gas that tend to migrate from the output zone 18 are returned to said output Zone by the flow of the condensable vapor of the working fluid 16, so that an equilibrium condition will be maintained. A sharp interface exists between the fluids 15 and 16, and heat discharge from the output zone 18 will not take place if said zone is occupied by said non-condensable gas 16.

Attention is now directed to FIGS. 2 and 3, wherein there is shown diagrammatically a means for controlling the heat output of a heat pipe by varying the position of the non-condensable gas therein. In these views the pipe section 10 is provided with an open end adjacent the heat output zone 18, and secured to said open end is a bellows 20 which is formed of a suitable heat-resistant material. In FIG. 2 the bellows is shown compressed, for forcing the non-condensable gas 15 into the pipe section for occupying the heat output zone thereof and preventing flow of condensable vapor 16 into said output zone, with the result that heat discharge from the pipe section will be arrested. In FIG. 3 the bellows is shown expanded. In this position the non-condensable gas 15 will be withdrawn from the pipe section, when the condensable vapor 16 will be allowed to conduct heat to the heat output zone 18 for discharge therefrom. As will be obvious, the bellows may be adjusted, either manually or automatically, for positioning the non-condensable gas as desired, for regulating the heat flow from the output zone 18. A satisfactory bellows adjusting means is shown in FIG. 5, to be described hereinafter.

To assure that stray molecules of the working fluid vapor will not condense in the bellows 20, the structure shown in FIG. 4 may be employed. Referring to this view, the pipe section 10 is open at its end adjacent the heat output zone and is provided with a coupling flange 21 which mates with a coupling flange 22 secured to the open inner end of the bellows 20. A suitable sealing gasket 23 is disposed between the flanges 21 and 22 and coupling bolts 24 secure the flanges and gasket in proper operative relationship for coupling the bellows to the pipe section. At its outer end, which is closed, the bellows 20 is secured to an actuator plate 25 by a clamping ring 26 and screws 27, the actuator plate having an actuator rod 28 for connection to a manual or an automatic actuator (not shown). Surrounding the bellows and secured to the outer end portion of the pipe section 10 is a casing 29 of thermal insulating material, and surrounding the bellows within the casing is a heating element 30 having terminals 31 for connection to a source of electric power.

In the arrangement shown in FIG. 4, the casing 29 and heating element 30 cooperate to maintain the bellows 20 at a temperature slightly higher than that of the pipe section 10 at the heat output zone 18 thereof, to prevent the aforementioned condensation in said bellows.

FIGS. 5 and 6 illustrate the invention as applied to a gravity-gradient stabilized, earth orbiting satellite and with means for automatically controlling the bellows. In these views a satellite body is shown at 33. A skirt is secured to the base of the body, and a layer of thermal insulation 34 is positioned adjacent said base. The skirt defines a sun shroud 35. The heat pipe of this embodiment of the invention is shown generally at 36 and includes a heat input portion 37 which eXtends into the interior of the satellite 'body and includes fins 38, and a heat output portion 39 which is positioned within the shroud and includes fins 40. At its outer end the output portion is provided with a bellows 41 which is similar to the bellows 20. The bellows 41 is mounted in a casing 42 of thermal insulating material, and includes an actuator rod 43. The output portion 39 is of angular configuration, as shown in FIG. 6. It should be understood, however, that said output portion may be of any desired configuration.

For controlling the output of the heat pipe of the embodiment of FIGS. 5 and 6, a heat-responsive device 44 is mounted within the satellite body 33. The heat responsive device may conveniently be of the type known as Vernatherm, manufactured by the American Radiator and Standard Sanitary Corporation. The heat responsive device and the heat input portion 37 of the pipe 36 are mounted in such a position within said body 33 that they will be exposed to heat from, say, operating electronic equipment such as is shown diagrammatically at 45. The heat responsive device 44 is operatively connected to the actuator rod 43 of the bellows 41 by a linkage 46 which is pivotally connected to the body 33 by a pin 47. The linkage 46 is shown as being of inverted L shape, but it should be understood that other configurations, or even a direct connection between the rod 43 and the device 44, may be used if desired.

The operation of the embodiment shown in FIGS. 5 and 6 is briefly as follows. An increase in temperature within the body 33 causes the heat responsive device 44 to extend, thus shifting the linkage 46 for extending the bellows 41 and withdrawing non-condensable gas in the heat pipe into said bellows, when condensable vapor will ow into the output portion 39 for discharge therefrom. The fins 38 and 40 function in their normal way to provide increased heat collection and discharge surfaces. When the temperature of the interior of the satellite body 33 has been lowered to a desired predetermined value, the heat responsive device 44 will operate to compress the bellows 41 for forcing non-condensable gas into the output portion 39 for limiting discharge of heat therefrom.

To assure proper operation of the invention as shown in FIGS 5 and 6, the earth-orbiting satellite may be gravity stabilized, so that the shroud will predominantly shield the output portion 39 of the heat pipe 36 from the sun. When used with a gravity stabilized satellite, the invention will maintain the interior of said satellite at a desired nearly constant temperature.

Referring now to the modification of the invention shown in FIG. 7, a satellite 49 is powered by a fuel capsule 50 which may conveniently be a radioactive isotope supplying heat to a thermoelectric generator. The fuel capsule is mounted in a suitable housing 51 having a base 52 of thermal insulating material, and said housing is connected to the satellite by legs 53. Fins 54 radiate heat generated by the heat capsule.

Mounted in the satellite 49 and extending into the housing is a heat pipe 55 which includes a heat input portion 56 of pancake configuration and a tubular heat output portion 57. As will be seen, the heat input portion 56 is disposed within the housing 51 to receive heat from the capsule 50, and the output portion 57, which extends through the base 52, is positioned to discharge heat to the interior of the satellite 49. Connected to the lower end of the heat pipe 55 is a bellows 58 which is mounted in an insulated enclosure 59. The bellows 58 is provided with an actuating rod 60 which is directly connected to a heat responsive device 61 that is mounted in the satellite. The heat responsive device may be the Vernatherm mentioned in the description of FIGS. 5 and 6 hereinabove.

As in the previously described modifications of the invention, the heat pipe 55 of FIG. 7 contains a non-condensable gas 62 and a condensable vapor 63 and a wick 64. Movement of the bellows by action of the heat responsive device 61 in accordance with heat requirements of the satellite will position the non-condensable gas in the output portion 57 for admitting condensed fluid from the heat input portion 56 to a relatively large or relatively small area of the output portion 57, whereby relatively large or relatively small amounts of heat generated by the fuel capsule will be transferred to the interior of the satellite, thereby maintaining some predetermined desired temperature.

In FIGS. 8 and 14 the invention is shown applied to a thermoelectric power generating system. In FIG. 8, which is partly diagrammatic, a heat pipe is shown at 65. The heat pipe 65 includes a sectionalized central heat input housing 66 which surrounds a radioactive isotope fuel capsule 67, and tubular heat output end portions 68 and 69. The outer ends of the portions 68 and 69 are open and have secured thereto bellows 70 and 71, respectively, the bellows being closed at their outer ends and ittedrwith actuating rods 72 and 73. A control device 74 is connectedV Vto the actuating rods V72 and 73Y by links 75 and 76 respectively, and in such a manner that movement of an actuator rod 77 of the device 74 in one direction will cause the bellows 70 to expand and the bellows 71 to compress. Movement of the actuator rod 77 in the opposite direction will, as will be obvious, bring about compression of the bellows 70 and expansion of the bellows 71.

Surrounding the heat output portions 68 and 69 are, respectively, thermoelectric arrays 78 and 79, said arrays being of well-known construction. The heat pipe 65, like the heat pipes of the other embodiments described hereinabove, is provided with a wick 80 which extends throughout the lengths of the output end portions 68 and 69 and adjacent the interior surfaces of the housing 66. Also as in the previously described embodiments of the invention, the heat pipe 65 has condensable and non-condensable uids therein. Surrounding the housing 66 is a thermal insulating jacket 82 to minimize heat loss.

It will be clear from the above description that expansion of one or the other bellows will withdraw non-condensable gas from its associated heat output portion for admitting condensable vapor thereto for transfer to the adjacent thermoelectric array for generating power. Thus one of the arrays will be on inoperative stand-by when the other is operating. The control device 74 is a conventional solenoid which, when the invention of this embodiment is mounted in a satellite, may be operated on command from the ground or another satellite. A command receiver and antenna therefor are shown diagrammatically at 83 and 84.

A variation of the application of the controllable heat pipe to the thermoelectric power generator, as shown in FIG. 8, is illustrated in FIG. 10 and may be preferable to insure reliable operation. Where applicable the numerals used in FIG. 8 are also employed in FIG. l0, in the interest of simplicity. There is some concern that the non-condensable gas, although initially divided into equal parts assigned to the left and right halves of the heat pipe, might become redistributed so that a preponderance permanently or temporarily would occupy one or the other half. This occurrence would interfere with proper operation. To preserve the desired distribution of equal portions of non-condensable gas in each half an annular metal bulkhead 86 is provided between the sections of the housing 66 to seal off the left and right halves of the heat pipe from each other. The wick 80 provides a continuous path for the liow of the liquid phase of the condensable iluid along the circular portion of one-half of the heat pipe, within one-half of the heat input housing and along the bulkhead 86. Heat transfer across the bulkhead to the other half of the heat pipe is accomplished by the same vaporization and condensation process as within a conventional heat pipe. If it is desired to shorten the length of the path of capillary dow, bridges 87 of wicking material, shown in dotted lines, can be provided, so long as care is taken to avoid obstructing seriously the ow of vapor from the input to the output ends of the pipe.

Another way of obtaining the same effect is illustrated in FIG. 1l, wherein, as in FIG. l0, the numerals of FIG. 8 are employed where applicable. FIG. 11 sho-ws in larger scale a modied annular bulkhead. In FIG. 11 the bulkhead, shown at 88, has two metal walls 89 and 90, is provided on each side with wicking material 91, and contains the vapor and liquid of a condensable iluid. The bulkhead 88 does not contain non-condensable tiuid. The edges o-f the plates 89 and 90 are sealed, so that the above-mentioned gas isolation of the two halves of the main heat pipe is provided. By its construction the bulkhead 88 also functions as a heat pipe, thus providing the desired ease of heat flow from one to the other halves of the main heat pipe.

If the allowable dimensions do not permit suflicient heat transfer across the bulkhead 86, the input section of the housing 66 on the right in FIG. 10 will attain a higher temperature and pressure than that on the left, and the heat liberated from the fuel capsule 67 in the right half of the heat pipe will not aid the operation of thermal-electric array 78 on the left. Accordingly, as shown in FIG. 12, wherein the numerals of FIGS. 8 and are used where applicable, an annular sectional separately sealed heat pipe 92 is provided to transfer most of the heat from the electrically inoperative half (in FIG. l0 the right half) to the operative (left) half. By this means the above-mentioned temperature and pressure difference is prevented and nearly all available heat is transferred to the operating thermal-electric array. Since a heat pipe is equally capable of transmitting heat in either direction, movement of actuator 77 (FIG. 8) to the position opposite to that shown does not impair the above uniformity of temperature and pressure and the utilization of available heat. The annular heat pipe 92 includes a sealed hollow body and is Similar to the heat pipe 65 to the extent that it contains wicking 93 and a condensable fluid 93a, the non-condensable fluid being omitted.

If desired, a cylinder and piston arrangement may be substituted for the bellows in each of the embodiments described hereinabove. Such an arrangement is shown in FIG. 9, wherein the heat pipe is shown at 94, `a cylinder attached to the outer end thereof at 95, and a piston in the cylinder at 96. A piston rod 97 connects the piston to a linkage, such as the linkage 46 shown in FIG. 5, or to the solenoid actuator 77. Other variations of this switching technique, such as switching among three or more outputs instead of the two outputs described, or switching partially rather than completely, are apparent.

The objective of the provision of a controllable heat pipe for the radioactive isotope fueled thermoelectric power generator is to obtain a better match in the useful operating life of the fuel capsule and of the thermoelectric array so as to attain an improvement in overall generator longevity and in economy of usage of isotope fuel. One way to attain this desired improvement is to provide means of exploiting a redundancy of thermoelectric arrays so that the presently imperfect reliability or the commonly experienced gradual degradation of thermo-electric arrays can be mitigated by directing the heat to a selected one of two or more alternative arrays, each array having been designed to sustain the entire electrical load by itself if in proper operating condition. Since the evolution of heat in konwn amount from a radioactive isotope fueled capsule is highly predictable and dependable, this provision will extend the probable life o-f the overall generator in proportion to the degree of redundancy.

Another Way of attaining improvement is to provide means of utilizing the changing thermal emission properties of the cheaper and more available relatively shortlived radioactive isotopes. If, as shown in FIG. 13, one array were removed and replaced with radiating fins and if provisions were made to operate the two bellows independently, then at the start of life with an excess of shortlived isotope the excess heat, which would other- 4wise destroy the array, could be harmlessly radiated to space by fully extending the bellows controlling the radiating fins. As the isotope progressively deteriorates in its heat output, the bellows is gradually compressed to reduce the amount of heat radiated to space and to maintain constant the amount of heat directed to the array. By sizing the bellows and radiating ns properly the required constant amount of heat can lbe provided to the array during several half-lives of the isotope.

More specifically, as shown in FIG. 13, a heat pipe similar to that illustrated in FIG. 8 is shown. The heat pipe of FIG. 13 includes a heat input housing 9S having a short-lived radioactive isotope 99 positioned therein. Insulation 100 surrounds the input housing. Heat output portions 101 and 102 extend from the opposite ends of the input housing, and connected to the outer ends of the heat output portions 101 and 102, respectively, are bellows 103 and 104. A thermoelectric generator 105 surrounds the heat output portion 101 and a iin assembly 106, comprising a plurality of radially directed fins, is fitted about the heat output portion 102. Actuators 107 and 108 are connected, respectively, to the bellows 103 and 104 through suitable linkages 109 and 110', and the actuators are connected to a receiver 112 which has an antenna 113 connected thereto. The receiver is responsive to pulse coded signals so that the actuators may be operated independently or simultaneously, for controlling the bellows.

The heat pipe 98 is provided with wicking 114 and condensable and non-condensable gases 115 and 116, respectively. Thus, on expansion of the bellows 104, the non-condensable gas will be drawn into said bellows, allowing the heat-conducting condensable gas to enter the heat output portion 102. Heat conducted by the condensable gas will be radiated by the n assembly 106. Conversely, as the heat output of the isotope deteriorates, the bellows 104 may be compressed, for limiting heat radiation by the iin assembly 106. Output of the thermoelectric generator may likewise be controlled by operation of the bellows 103, for admitting a desired amount of condensable gas into the heat output por tion 101.

It is apparent from the above descriptions that by the use of redundant arrays, several branches to the heat pipe and excess heat radiating provisions, the combination of features affording long array life with relatively short-lived isotopes can be employed to advantage.

If desired, the cylinder and piston arrangement shown in FIG. 9 may be substituted for the bellows in each of the embodiments described hereinabove.

Other variations of this switching technique, such as switching among three or more outputs instead of the two outputs described, or switching partially rather than completely, are apparent.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood at this time that within the scope of the appended claims, the invention may be practiced otherwise than as specically described.

What is claimed is:

1. A controllable heat pipe comprising:

a pipe section having a heat input end portion and a heat output end portion, the heat input end portion of the pipe being positioned for exposure to a source of heat external of the pipe and the heat output end portion of said pipe being positioned to discharge such heat to an area remote from the heat input end portion of the pipe,

fluid conducting means in the pipe section,

a condensable uid in the pipe section and movable as vapor under the iniiuence of heat toward the heat output end portion of the pipe,

a non-condensable gas in the pipe section and limiting flow of said vapor toward said output end portion, and

volumetric means connected to the pipe section at the heat output end portion thereof and adjustable for withdrawing therefrom and returning thereto noncondensable gas whereby heat conducted by the vapor will be permitted to flow toward and away from said output end portion of the pipe for discharge therefrom,

said fluid conducting means returning as liquid the vapor condensed at the heat output end portion of the pipe to the heat input end portion thereof.

2. A controllable heat pipe as recited in claim 1,

wherein said uid conducting means in the pipe section comprises wicking material.

3. A controllable heat pipe as recited in claim 1,

wherein said last-mentioned means comprises a bellows.

4. A controllable heat pipe as recited in claim 1, wherein `said last-mentioned means comprises a cylinder connected to the output end portion of the pipe section, and

a piston movable in the cylinder.

5. A controllable heat pipe as recited in claim 3, including additionally heat-responsive actuating means positioned in the environment of the input end portion of the pipe section, and

means operatively connecting the heat-responsive actuating means to the bellows.

6. In combination with a heat source,

a controllable heat pipe including a pipe section having a heat input end portion positioned for exposure to the heat source and a heat output end portion positioned remote from said heat source,

a condensable gas in the pipe section and movable by heat from the source toward the heat output end portion of the pipe where the heat of condensation is released,

a non-condensable gas in the pipe section and limiting flow of said condensable gas toward said output end portion of the pipe,

a bellows connected to the heat output end of the pipe section,

heat-responsive actuating means positioned adjacent the heat source,

said actuating means so positioning the bellows that the amount of non-condensable gas admitted to the heat output end portion of the pipe will be regulated, whereby the area surrounding the input end portion of the pipe section will be maintained at a nearly constant temperature, and

Huid conducting means in the pipe section for returning as liquid the condensable gas condensed at the heat output end portion of the pipe to the heat input end portion'thereof.

7. The combination recited in claim 6, wherein the conducting means in the pipe section consists of wicking material.

8. The combination recited in claim 6,

includingv additionally link means connected between the bellowsl and the actuating means.

9. The invention as recited in claim 1, wherein the condensable uid is water, and the non-condensable gas is hydrogen.

10. The invention as recited in claim 1, wherein the condensable uid is ethyl alcohol, and the noncondensable gas is hydrogen.

11. The invention as recited in claim 1, wherein the condensable lluid is sodium gas, and the noncondensable gas is hydrogen.

12. The invention as recited in claim 1, wherein the condensable Huid is water, and the non-condensable gas is air.

13. The invention as recited in claim 1, wherein the condensable iiuid is ethyl alcohol, and the noncondensable gas is air.

14. The invention as recited in clai-m 6,

including additionally an insulated casing enclosing the bellows, and

heat means surrounding the bellows within the casing for preventing condensation within the bellows of any stray molecules of condensable gas.

15. In combination with a space satellite having heat producing means therein,

a controllable heat pipe comprising:

a pipe section having a heat input end portion mounted within the satellite and in the environment of the heat producing means and a heat output portion mounted exteriorly of the satellite,

a condensable gas in the pipe section and movable by heat from the heat producing means toward the heat output end portion of the pipe,

a non-condensable gas in the pipe section and limiting flow of said condensable gas toward said output end portion,

volumetric means connected to the pipe section at the heat output end portion thereof and exteriorly of the satellite and being adjustable for withdrawing therefrom and returning thereto non-condensable gas whereby heat conducted by the condensable gas will be permitted to flow toward and away from said output end portion of the pipe for discharge exteriorly of the satellite, and

fluid conducting means in the pipe section for returning liquid produced by condensation of the condensable gas at the heat output end portion of the pipe section to the heat input end portion thereof.

16. The combination recited in claim 15, wherein the satellite has a body, a shroud extending below the body and a wall of insulating material between the body and the shroud.

wherein the heat producing means and the heat input end portion of the pipe section are mounted within the body, and

wherein the heat output portion of the pipe section and the second mentioned means are mounted exteriorly of the body adjacent the wall of insulating material and within the shroud.

17. In combination with a satellite having a body,

a heat generating fuel capsule, and means insulating the fuel capsule from the body,

a controllable heat pipe including a pipe section having a heat input end portion mounted in the environment of the fuel capsule and a heat output portion extending through the insulating means and within the body,

a condensable gas in the pipe section and movable by heat toward the heat output end portion thereof,

a non-condensable gas in the pipe section and limiting flow of said condensable gas toward said output end portion,

volumetric means connected to the pipe section at the heat output end portion thereof and adjustable for withdrawing therefrom and returning thereto noncondensable gas whereby heat conducted by the condensable `gas will flow into and away from said output end portion of the pipe for discharge therefrom,

means for actuating said last-mentioned means in response to temperature changes in the body, and

liquid conducting means in the pipe section and returning as liquid condensable gas condensed at the heat output end portion of the pipe section to the heat input portion thereof.

18. In combination with a heat generating fuel capsule, and a pair of thermoelectric generators,

a controllable heat pipe including a pipe section having a centrally located heat input housing and heat output end portions,

said housing having said capsule therein,

one of said heat output end portions being positioned adjacent each of said generators,

a Ibellows connected to the outer end of each of said heat output end portions,

a condensable gas in the pipe section and movable by heat from the capsule toward the heat output end por-,tions of the pipe section,

a non-condensable gas in the pipe section and limiting liow of said condensable gas toward said output end portions,

means connected to said bellows for alternately expanding one thereof while compressing the other for alternately withdrawing non-condensable gas from one output end portion and forcing non-condensable gas into the other said output end portion, the with-drawal of non-condensable gas from one output end portion permitting the flow of condensable gas therein, whereby heat wil be transferred to the thermoelectric generator adjacent thereto for generating electric power, the forcing of non-condensable gas into the other of said heat output end portions arresting flow of condensable gas into said other heat output end portion for limiting heat transfer to the thermoelectric generator adjacent thereto, and

conducting means in the pipe section for returning as liquid the condensable gas condensed in the heat output end portions of said pipe section to the heat input housing thereof.

19. The combination recited in claim 18,

wherein the heat input housing comprises a pair of mating sections, and

including additionally a bulkhead between the housing sections and dividing the heat pipe into independently operating halves,

said bulkhead conducting heat between said halves.

20. The combination recited in claim 18,

wherein the heat input housing comprises a pair of mating sections, and

including additionally an annular heat pipe between the housing sections and dividing the heat pipe into independently operating halves,

said annular heat pipe conducting heat between said halves and including a pair of spaced walls cooperating with said housing sections to dene a closed chamber, wicking material between the walls, and a condensable gas in the chamber.

21. The combination recited in claim 18,

wherein the heat input housing comprises a pair of mating sections, and

including additionally an annular heat pipe surrounding the housing sections and conducting heat between the sections,

said annular heat pipe having wicking and a condensable gas therein.

22. A controllable heat pipe including a pipe section having a centrally located heat input housing and rst and second heat output end portions,

a fuel capsule in the housing,

a thermoelectric generator on said rst heat output end portion,

a n assembly on said heat output end portion,

a rst bellows connected to the outer end of said rst heat output end portion,

a second bellows connected to the outer end of said second heat output end portion,

a condensable gas in the pipe section and movable by heat produced by the capsule from the housing toward said heat output end portions,

a non-condensable gas in the pipe section and limiting flow of said condensable gas toward said heat output end portions,

means for actuating said rst bellows for regulating the ow of condensable gas into said first heat output end portion for controlling heat discharge therefrom to said thermoelectric generator,

means for actuating said second bellows for regulating the flow of condensable gas into said second heat output end portion for controlling heat discharge therefrom to said fin assembly, and

means for supplying control signals to said penultimate andlast mentioned means for operating the same independently, whereby the heat supplied by the fuel capsule to the thermoelectric generator may be maintained substantially constant.

23. A controllable heat pipe including a pipe section having a centrally located heat input housing and rst and second heat output end portions,

a fuel capsule in the housing,

a thermoelectric generator on said rst heat output end portion,

a fin assembly on said heat output end portion,

a rst bellows connected to the outer end of said first heat output end portion,

a second bellows connected to the outer end of said second heat output end portion,

a condensable gas in the pipe section and movable by heat produced by the capsule from the housing toward said heat output end portions,

a non-condensable gas in the pipe section and limiting flow of said condensable gas toward said heat output end portions,

means for actuating said first bellows for regulating the llow of condensable gas into said rst heat output end portion for controlling heat discharge therefrom to said thermoelectric generator,

means for actuating said second bellows for regulating the ow of condensable gas into said second heat output end portion for controlling heat discharge therefrom to said 1in assembly, and

means for supplying control signals to said penultimate and last mentioned means for operating the same simultaneously, whereby the heat supplied by the fuel capsule to the thermoelectric `generator may be maintained substantially constant.

References Cited UNITED STATES PATENTS 2,581,347 1/1952 Backstrom 165-105 X 2,924,635 2/1960 Narbut 16S-105 X 2,711,882 6/ 1955 Narbutovskih 16S-47 2,961,476 11/1960y Maslin et al. 174-15 3,371,298 2/1968 Narbut 336-57 3,229,759 1/1966 Grover 165-105 3,330,130 7/1967 Schraith et al. 62-259 3,332,476 7/1967 McDougal 16S-105 OTHER REFERENCES Cotter, T. P.: Theory of Heat Pipes, Los Alamos Scientic Laboratory, Los Alamos, N.M., March 1965, pp. 1 and 33, LA-3246-MS.

Cotter, T. P.: Heat Pipes, Los Alamos Scientifiic Laboratory, LA-3246-MS,'March 1965 pp. 1-33.

Deverall, I. E. and Kemme, J. E.: High Thermal Conductance Devices Utilizing the Boiling of Lithium or Silver, Los Alamos Scientific Laboratory (LA-3211), Apr. 9, 1965, page 37.

ROBERT A. OLEARY, Primary Examiner A. W. DAVIS, IR.,Assistant Examiner 

